Hi again, On Wed, 5 Nov 2003, Patty Fong wrote:
Hi to anyone reading this. i'm still strugling a bit with data type declarations.
The was i understand it is that if i delcare a new data type:
data myType = myType a | b | c
This isn't entirely correct. The names of types have to begin with capital letters, as do constructors (more later). So you would need this to be: data MyType = MyType A | B | C where A is an existing type. This type now has three constructors: MyType :: A -> MyType B :: MyType C :: MyType It's perhaps a bit easier to understand when the names are different. When we say: data Foo = Bar Int | Baz String this means that a "Foo" is of one of two forms (the "|" can be read as disjunction). A value of type Foo is either of the form "Bar x" for some x which is an Int or "Baz y" for some y which is a String. "Bar" and "Baz" are called "constructors" because they take arguments and "construct" a Foo. So, in this case, Bar :: Int -> Foo Baz :: String -> Foo are the two constructors. Of course, you have have any number of constructors and each can have any number of arguments. data Foo = Bar | Baz Int | Bazaa String Bool Now there are three constructors: Bar :: Foo Baz :: Int -> Foo Bazaa :: String -> Bool -> Foo they can be recursive: data Foo = Bar | Baz Int Foo Bar :: Foo Baz :: Int -> Foo -> Foo and can have "type variables", for instance: data Foo a = Bar | Baz a here, something of type "Foo a" is either of the form Bar or of the form Baz x for some x which is of type a. This has constructors: Bar :: Foo a Baz :: a -> Foo a I hope this sheds some light on the issue... -- Hal Daume III | hdaume@isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume